Police tactical units and how they work.

A Florida police sniper killed a would-be bank robber on Tuesday night while he attempted to flee the scene with a cohort and a hostage. The criminals tried to leave the bank after a SWAT team entered the back of the building with a concussion grenade. The team had surrounded the building about 10 hours earlier, after a bank employee called 911 to report the robbery. How do police SWAT teams work?

They get called in when regular police are out of their depth. Since the 1960s, law enforcement agencies have trained small teams of officers for dangerous situations and outfitted them with military equipment. (The letters of SWAT stand for "Special Weapons and Tactics.") Teams are often called in when a gunman barricades himself into a building, especially if he takes hostages. They may also help regular police serve warrants, search for a dangerous criminal, or control large crowds.

Most SWAT personnel are assigned one of three roles—negotiator, rifleman, or entry officer. For a typical hostage situation, the riflemen deploy to sniping positions in pairs while the entry team contains the area. As the negotiators work from the SWAT command post, some members of the entry team might prepare for an emergency assault on the hostage-takers, called a "crisis entry."

Team members train to use submachine guns, sniper rifles, and assault weapons. They may also have access to concussion grenades, armored vehicles, or even a trained monkey for special circumstances. To get on a team, a police officer must have a certain amount of experience—usually three years—and then be able to pass several rounds of tests. He's run through an obstacle course and then tested on his weapons skills and tactical knowledge. (Some agencies also require a psychological evaluation.) Those who make it onto the team typically get a few hundred dollars extra per month in hazard pay.

SWAT teams are ubiquitous, and they vary in size and organization. By the mid-1990s, more than 80 percent of American cities had active teams, as did more than half of all law enforcement agencies in the country with more than 50 officers. (There are even SWAT teams for the U.S. National Park Service and for one of the subway systems in the Bay Area.) The LAPD has about 60 people on its team, along with six sergeants and one lieutenant who runs the show. By contrast, the Houston Police Department has only 24 full-time officers on its team, with an auxiliary group of part-timers who specialize in containment. In Florida, all SWAT officers are part-time. (A part-timer works a regular police job but has to report for duty whenever he gets a page or text message from his commander. He'll also attend regular SWAT training sessions.)

To call up a SWAT team, an officer can contact a police dispatcher who then passes along the request—and the details of the situation—to the SWAT commander. The commander can then decide to send her entire team or some part of it. If her team is overwhelmed, or if a hostage situation persists for so long that her officers get tired, she can ask for help from a nearby jurisdiction.

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Explainer thanks Detective David Arnott of the Florida SWAT Association and David Klinger of the University of Missouri. Thanks also to Paige Rossetti for asking the question.